Postscript viewer:
To view the Postscript files
(lecture notes, homework asssignments and old exams)
on your PC screen or to print it out on a printer,
you need to install
the GSview from here.

PDF viewer:
To view the PDF files on your PC screen, you may need to install
the Acrobat Reader from
Adobe.
(Other softwares, such as GSview, can be possible alternatives.)
Acrobat Reader can be downloaded from the
Adobe site